In situations where an output transistor is used to drive a load, it is frequently desirable to ensure that the output transistor does not carry excessive current. If a short circuit should appear across the load, the resulting high current could damage the output transistor.
The way in which protection is achieved for the output transistor depends on whether the transistor is bipolar type or an FET. For bipolar transistors, complex protection circuitry is usually not required because its gain decreases rapidly at high levels of collector current. This decrease in gain tends to limit collector current to a safe level if an adequate heat sink is used and the "on" time of the transistor is sufficiently small.
For FET's, a different approach is required. This is because FET's can carry a very high current when they are turned fully on. Consequently, a more elaborate protection scheme is required in order to limit the current of the FET to a safe level when the load becomes short-circuited. Such a scheme has typically included circuitry for sensing drain current and for turning off the FET before its current level becomes destructive. These schemes perform adequately, but their extra complexity and cost are undesirable.